The Union of Lublin (Belarusian: Лю́блінская ву́нія; Polish: Unia lubelska) - signed July 1, 1569 in Lublin, united the Kingdom of Poland and the Grand Duchy of Lithuania into a single state, the Polish-Lithuanian Commonwealth with official name: Rzeczpospolita Obojga Narodów (The Republic of Both Nations in Polish). The Commonwealth was ruled by a single elected monarch who took the duties of the Polish king and the Lithuanian grand duke, and governed by a common diet and senate. The Union was necessitated by Lithuania's position in wars with Russia.
The Grand Duchy of Lithuania maintained a good deal of sovereignty, administering its own laws, government, finances, and army. Lithuania gave up, however, the territories of Podlachia, Volhynia, Podolia and the Kyiv region.
Essentially, the Union of Lublin created the largest ever state in Europe's history, before the arrival of European Union. Many historian also consider the Union of Lublin to have created a similar state to present-day European Union, thus considering the Union to be kind of a predecessor of the Maastricht treaty. The former, however, created a state of countries more deeply linked than the present-day EU.
The drafters of the Union of Lublin expected that the countries of Lithuania and Poland should be linked together more closely than they actually were, because the Second statute of Lithuania had not lost its power, and some of its provisions substantially differed from the acts of Union of Lublin.
Many historians also consider the Union of Lublin to have created a similar state to the present-day European Union, thus considering the Union to be kind of a predecessor of the Maastricht treaty.